Yea just using the battery already in the boat. Just run one long power cord from the head unit to the battery?

You could put a jumper on the purple wire from your ignition but most of our boats are old and there is a pretty big voltage drop. My radio will only go to about 75% before it kicks off from lack of power. If you run a new wire all the way back to the battery, you won't have to worry about old wire issues.

There is a ACC and Ground wire coming off the key side of the dash, I just tied my headunit into that just last night and it works great. Mine were pinned up underneath the dash and are labelled right on the wires as to what they are.